Edit book: Add a shortcut (Ctrl+Alt+Down) to edit the next file in the book spine. Fixes #1779616 [Editor: Add a keyboard shortcut to open the next file](https://bugs.launchpad.net/calibre/+bug/1779616)

src/calibre/gui2/tweak_book/boss.py

@@ -1451,6 +1451,9 @@ class Boss(QObject):
                 _('Editing files of type %s is not supported' % mime), show=True)
         return self.edit_file(name, syntax)
 
+    def edit_next_file(self, backwards=False):
+        self.gui.file_list.edit_next_file(self.currently_editing, backwards)
+
     def quick_open(self):
         if not self.ensure_book(_('No book is currently open. You must first open a book to edit.')):
             return

src/calibre/gui2/tweak_book/file_list.py

@@ -734,6 +734,21 @@ class FileList(QTreeWidget):
             error_dialog(self, _('Cannot edit'),
                          _('No item with the name: %s was found') % name, show=True)
 
+    def edit_next_file(self, currently_editing=None, backwards=False):
+        category = self.categories['text']
+        seen_current = False
+        items = (category.child(i) for i in xrange(category.childCount()))
+        if backwards:
+            items = reversed(tuple(items))
+        for item in items:
+            name = unicode(item.data(0, NAME_ROLE) or '')
+            if seen_current:
+                self._request_edit(item)
+                return True
+            if currently_editing == name:
+                seen_current = True
+        return False
+
     @property
     def all_files(self):
         return (category.child(i) for category in self.categories.itervalues() for i in xrange(category.childCount()))
@@ -982,6 +997,7 @@ class FileListWidget(QWidget):
         for x in ('delete_done', 'select_name', 'select_names', 'request_edit', 'mark_name_as_current', 'clear_currently_edited_name'):
             setattr(self, x, getattr(self.file_list, x))
         self.setFocusProxy(self.file_list)
+        self.edit_next_file = self.file_list.edit_next_file
 
     def build(self, container, preserve_state=True):
         self.file_list.build(container, preserve_state=preserve_state)

src/calibre/gui2/tweak_book/ui.py

@@ -337,6 +337,11 @@ class Main(MainWindow):
         self.action_open_book = treg('document_open.png', _('&Open book'), self.boss.open_book, 'open-book', 'Ctrl+O', _('Open a new book'))
         self.action_open_book_folder = treg('mimetypes/dir.png', _('Open &folder (unzipped EPUB) as book'), partial(self.boss.open_book, open_folder=True),
                                             'open-folder-as-book', (), _('Open a folder (unzipped EPUB) as a book'))
+        self.action_edit_next_file = treg('arrow-down.png', _('Edit &next file'), partial(self.boss.edit_next_file, backwards=False),
+                'edit-next-file', 'Ctrl+Alt+Down', _('Edit the next file in the spine'))
+        self.action_edit_previous_file = treg('arrow-up.png', _('Edit &previous file'), partial(self.boss.edit_next_file, backwards=True),
+                'edit-previous-file', 'Ctrl+Alt+Up', _('Edit the previous file in the spine'))
+        # Qt does not generate shortcut overrides for cmd+arrow on os x which
         # Qt does not generate shortcut overrides for cmd+arrow on os x which
         # means these shortcuts interfere with editing
         self.action_global_undo = treg('back.png', _('&Revert to before'), self.boss.do_global_undo, 'global-undo', () if isosx else 'Ctrl+Left',